Field of the Invention:
The invention relates to an apparatus for producing an extrusion or billet having a central opening, from a deformable composition.
In many fields of work, there is a need for shorter or longer material extrusions with a central opening or longitudinal conduit, that is hollow-cylindrical bodies of material. After an insertion of an optical wave guide into the relatively narrow longitudinal conduit, such material extrusions are used as optical waveguide plug prongs or pins. In principle, sheaths or sockets for optical waveguides have the same structure, in that they are once again hollow cylindrical bodies. Such prong bodies and sheaths can be made by extrusion from a soft, deformable composition, such as a ceramic-like zirconium oxide. After the extrusion, care is taken to ensure that the composition will set and still maintain its hollow-cylindrical shape.
It is of paramount importance to ensure that the hollow-cylindrical bodies of material that are produced have good concentricity of the central conduit. Meeting that condition makes high demands of the tool used for the extrusion, particularly if the central conduit is supposed to be quite narrow. For instance, in the case of the aforementioned optical waveguide plug prongs or pins, hollow-cylindrical material extrusions having an outside diameter of 2.5 to 2.6 mm and a conduit diameter of only 80 to 100 .mu., are needed. The location of the conduit cannot be allowed to deviate by more than 15 to 30 .mu. (tolerance) from the geometric center. The situation is similar for optical waveguide sheaths or sockets, which, for example, should have an outside diameter of 4.0 mm and a conduit diameter of 2.5 mm. The tool should also be constructed in such a way that the soft composition does not collapse inward immediately after the extrusion process, which would cause the central conduit to "stick together". It would also be desirable for one and the same tool to be usable for producing material extrusions with central conduits of various diameters.
In principle, a tool could be constructed that operates out of an outlet opening with a mandrel disposed in front of it, during the extrusion of the material. However, in the case of especially small conduit diameters, the diameter of such a mandrel must be no greater than approximately the thickness of a hair. Experiments with a tool that is constructed in this way have shown that the central adjustment of the mandrel entails time-consuming work under the microscope. Moreover, problems arise particularly upon startup of operation, since the first time there is a flow toward the outlet opening, the composition flows unevenly along the mandrel, causing the mandrel to bend (resulting in a non-concentric longitudinal conduit) or even break off. In order to overcome that drawback, other experiments were made with a tool that used a rotating mandrel. In that case, however, it was found that the rotary speed had to be very high, for example 20,000 to 30,000 rpm, which necessitated an expensive drive mechanism. Moreover, the prong became hot during operation because of the friction with the deformable composition, with undesirable effects. Thus the tool must not only be adequately sturdy, but it must not impair the flow behavior of the flowing composition.